Manhunt for London bomber as threat level raised

Police forensics officers works alongside an underground tube train at a platform at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. British police are treating an incident on a London Underground train on Friday as an act of terrorism, saying "a number of people" had been injured. Twenty-two people were injured after a bomb blast on a packed London Underground train on Friday, the National Health Service said in a statement. (Adrian DENNIS / AFP)

(AFP) - British police raced Saturday to track down whoever planted a bomb on a packed London Underground train, as authorities raised the national threat level to maximum, meaning another attack may be imminent.

Friday morning's blast, which left 29 people injured, was Britain's fifth terror attack in six months and was claimed by the Islamic State group.

Prime Minister Theresa May announced late Friday that the threat level had been raised to "critical", and said troops would take over guarding key sites to free police officers for deployment elsewhere.

The explosion at Parsons Green station in south-west London sparked a "wall of fire" that left passengers with burns and caused a stampede of panicking people in which some were trampled.

No-one has yet been arrested over the bombing, but anti-terrorism police chief Mark Rowley said the investigation was making "really good progress".

"We're chasing down suspects," he told reporters.

"Somebody has planted this improvised explosive device on the Tube. We have to be open-minded at this stage about him and potential associates."

May said military personnel would take over guard duty at certain closed "protected sites", freeing up 1,000 police officers to be deployed on the transport network and on streets across Britain.

The country was last on critical alert after the bombing at a concert in Manchester in May, which was also claimed by the IS group.

In a statement Friday, the group said an IS "detachment" had carried out the London attack.

Rowley earlier said most of the injuries were due to "flash burns", while others were wounded as passengers ran out of the station in panic.